It is proposed to create a method and the apparatus needed to enable setting the optimum rate for an implanted pacemaker. The method is applicable to fixed-rate, as well as exercise-responsive pacemakers and employs continuous measurement of stroke volume and ejection fraction, detected by the impedance change meaasured between a pair of electrodes on the pacing catheter in the right ventricle. The stroke volume impedance change (SVDeltaZ) is converted to true stroke volume (SV) by a single measurement of cardiac output using a single intravenous injection of dilute saline as the indicator. Additional electrodes on the pacing catheter are used to detect the saline dilution curve. The dilution-curve electrodes are located between the stroke-volume measuring electrodes on the right-ventricular (pacing) catheter. The dilution curve is a record of the change in resistivity of right-ventricular blood following the single intravenous injection of dilute saline indicator. The dilution curve is easily calibrated electrically by knowing the manner in which blood resistivity changes (Deltap) in response to a change of saline concentration (Deltac); the quantity Deltap/Deltac is specific for a given species. We have already obtained such data for dog and man. Cardiac output is then divided by heart rate to obtain true stroke volume (SV). Thus, the SVDeltaZ is calibrated in terms of true SV; now stroke volume is available on a beat-by-beat basis. Ejection fraction will be determined from the ratio of SVDeltaZ to Zd, where Zd is the right ventricle. The method of identifying the optimum pacing rate involves plotting cardiac output (SVDeltaZ multiplied by pacing rate) versus pacing rate. Such a curve rises to a plateau with increasing pacing rate. The optimum pacing rate is the lowest that brings the curve to the plateau. Nothing is gained by pacing at a higher rate. In dogs with A-V block, cardiac output and ejection fraction will be derived from the right-ventricular impedance change. Cardiac output will be plotted versus pacing rate. Ejection fraction (SVDeltaZ/Zd) will also be plotted versus pacing rate to assess its value to identify the optimum pacing rate. The procedure will be studied in normal resting and paced dogs and dogs exercising moderately and vigorously on the treadmill.